netdev - Re: [PATCH v2 net] tcp: randomize timestamps on syncookies

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Message-ID: <1493991983.7796.40.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Fri, 05 May 2017 06:46:23 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2 net] tcp: randomize timestamps on syncookies

On Fri, 2017-05-05 at 11:36 +0200, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> > 
> > Whole point of randomization was to hide server uptime, but an attacker
> > can simply start a syn flood and TCP generates 'old style' timestamps,
> > directly revealing server jiffies value.
> > 
> > Also, TSval sent by the server to a particular remote address vary
> > depending on syncookies being sent or not, potentially triggering PAWS
> > drops for innocent clients.
> > 
> > Lets implement proper randomization, including for SYNcookies.
> 
> 
> Thanks a lot Eric, this works for me (I also tested ipv6 this time ;) )
> 
> Minor nit:
> net/ipv4/tcp_ipv4.c:154:6: warning: unused variable 'seq' [-Wunused-variable]
> 

Thanks Florian, I will remove this in v3, and add your tags.

> Other than this:
> Reviewed-by: Florian Westphal <fw@...len.de>
> Tested-by: Florian Westphal <fw@...len.de>